In a conventional pneumatic tire of the “tubeless” type (that is to say of the type without an inner tube), the radially internal face comprises an airtight layer (or more generally a layer that is impermeable to any inflation gas) which enables the pneumatic tire to be inflated and kept under pressure. Its airtightness properties enable it to guarantee a relatively low rate of pressure loss, making it possible to keep the tire inflated, in the normal operating state, for a sufficient time, normally several weeks or several months. It also has the role of protecting the carcass reinforcement from diffusion of air coming from the internal space of the tire.
This role of airtight inner layer or “inner liner” is today essentially fulfilled by compositions based on an elastomer or butyl rubber, long renowned for their excellent airtightness properties.
Moreover, in recent years, tire manufacturers have made particularly strenuous efforts to develop novel ways of solving a problem dating back from the very start of the use of wheels fitted with inflated tires, namely how to allow a vehicle to continue to travel despite a substantial or total loss of pressure of one or more tires. For decades, the spare wheel was considered to be the only and universal solution. Then, more recently, the considerable advantages of possibly dispensing with the spare tire have appeared. The concept of “extended mobility” was developed. The associated techniques allow the vehicle to run with the same tire, dependent on certain limitations to be respected, after a puncture or a pressure drop. This makes it possible for example to get to a point of repair without having to stop, often in hazardous circumstances, to fit the spare wheel.
Self-sealing compositions that can achieve such an objective, which by definition are capable automatically, i.e. without any external intervention, of sealing a tire in the event of it being punctured by a foreign body, such as a nail, are among the technical solutions which were studied. They are particularly difficult to develop, having to satisfy many conditions of a physical and chemical nature. They must in particular be effective over a very wide operating temperature range and over the entire lifetime of the tires.
This puncture-resistance role is fulfilled by self-sealing compositions which are themselves also usually based on butyl rubber. As examples, U.S. Pat. No. 4,113,799 (or FR 2 318 042) describes, as self-sealing layer, a composition comprising a combination of partially crosslinked butyl rubbers of high and low molecular weights, possibly in the presence of a small portion of a thermoplastic styrene elastomer. U.S. Pat. No. 4,228,839 has proposed a rubber compound containing a first polymer material that degrades when irradiated, such as polyisobutylene, and a second polymer material that crosslinks when irradiated, preferably a butyl rubber. U.S. Pat. No. 4,426,468 has also proposed a self-sealing composition based on crosslinked butyl rubber of very high molecular weight.
A known drawback of butyl rubbers is that they suffer large hysteresis losses, furthermore over a wide temperature range, which drawback has repercussions on the layers or compositions themselves, whether they are of gastight type or of the self-sealing type, giving them a large increase in hysteresis and considerably degrading the rolling resistance of tires using such compositions.